# Variable Amplitude sine wave

11 views (last 30 days)
Margarita on 2 Jul 2014
Edited: Shravankumar P on 2 Jul 2014
Hello, I am trying to learn some more about the function fft, for that I have a code like this
%Time specifications:
Fs = 1000; % samples per second
dt = 1/Fs; % seconds per sample
StopTime = 0.6; % seconds
t = (0:dt:StopTime-dt)'; % seconds
% Sine wave:
Fc = 500; % hertz
x = sin(2*pi*Fc*t);
% Plot the signal versus time:
figure;
plot(t,x);
xlabel('time (in seconds)');
title('Signal versus Time');
zoom xon;
Now I dont understand why, but the output of the plot is a sinusoidal wave that increases in amplitud. why is that? As far as my understanding goes, that should not happen. I know that it has something to do with the Fs value. but what is the relationship exactly. How should I pick a good Fs value to avoid this?
Thank you very much

#### 1 Comment

Shravankumar P on 2 Jul 2014
you need to change the frequncy Fc ; try Fc=10 to 50 ;
or you need to change the Fs, eg: Fs=100000;

Mischa Kim on 2 Jul 2014
Edited: Mischa Kim on 2 Jul 2014
Margarita, that's because the dt value is too large. Try
dt = 1e-6;
instead and you should receive a proper sinusoidal signal with a constant amplitude of 1. As far as next steps are concerned, the optimal value of dt depends on your particular application, whatever you need to do.

Margarita on 2 Jul 2014
Thank. But why is the dt generating that behaviour?
Mischa Kim on 2 Jul 2014
The frequency of the signal is 500 Hz, the period is therefore 1/500 s which is exactly 2*dt. So you evaluate the sine pretty much dead on at its zero crossings. That's the reason why the signal amplitudes in your plot are tiny (1e-13).
Margarita on 2 Jul 2014
Thank you